BCH2022 Chap.8 Fatty Acid and Complex Lipid Synthesis
Fatty Acid and Complex Lipid Synthesis
Fatty acid synthesis moves acetyl carbon from mitochondria to cytosol through citrate, forms malonyl-CoA in an energy-requiring committed step and uses NADPH during repeated chain elongation. Its compartment, carriers and carbon donor differ from beta-oxidation, so it cannot be described as the same pathway in reverse.
Malonyl-CoA also helps restrain long-chain mitochondrial entry, coordinating synthesis with reduced oxidation. Newly made fatty acids can be elongated, desaturated and incorporated into triacylglycerols or phospholipids. The correct product depends on whether the cell needs compact energy storage or amphipathic membrane material.
De novo fatty-acid synthesis requires carbon export, energy and reducing power to converge in the cytosol. Mitochondrial acetyl-CoA is represented outside the organelle through citrate handling, after which acetyl units can support the committed formation of malonyl-CoA.
Acetyl-CoA carboxylase therefore sits at a useful control point: it consumes ATP, responds to nutritional and hormonal state, and produces the two-carbon donor used during chain elongation. Fatty-acid synthase repeatedly condenses and reduces those units, using NADPH rather than the NADH that commonly carries electrons from oxidative catabolism.
Abundant acetyl carbon alone is consequently insufficient if ATP, NADPH or enzyme activity is limiting. Malonyl-CoA also coordinates direction by restraining long-chain fatty-acid entry into mitochondria, helping to prevent high synthesis and oxidation at the same time. Compare the two pathways by purpose, compartment, carrier, carbon donor and products instead of describing synthesis as beta-oxidation backwards.
Palmitate from the core sequence can be modified further, but the biological question often continues into complex-lipid assembly. Triacylglycerol provides hydrophobic energy storage, whereas phospholipids are amphipathic membrane components; product choice follows cellular purpose and backbone availability. For evidence, separate total lipid amount, class composition and newly synthesised lipid.
A changed phospholipid profile could arise from synthesis, remodelling or degradation. Labelled incorporation over a defined interval, normalised to cell number or protein, tests new synthesis more directly. Pair it with malonyl-CoA or acetyl-CoA-carboxylase state to connect signal to mechanism.
In an integrated answer, follow fed-state carbon from glucose through citrate export and then state why the same regulatory state restrains opposing mitochondrial oxidation.
What this chapter covers
- 01
Citrate carbon export
- 02
Acetyl-CoA carboxylase
- 03
Malonyl-CoA
- 04
NADPH use
- 05
Palmitate synthesis
- 06
Complex lipid assembly
Worked example · free
Compare synthesis with oxidation
- +1State that acetyl carbon is exported through citrate and recovered in the cytosol.
- +1Identify malonyl-CoA formation and NADPH as key requirements for synthesis.
- +1Explain that rising malonyl-CoA restrains long-chain mitochondrial entry and reduces opposing oxidation.
- +1Route the product into triacylglycerol when the objective is energy storage.
Key terms
- Citrate shuttle
- The citrate shuttle transfers acetyl carbon from mitochondria to the cytosol for biosynthesis.
- Malonyl-CoA
- Malonyl-CoA is the activated two-carbon donor used repeatedly during fatty acid elongation.
- Acetyl-CoA carboxylase
- Acetyl-CoA carboxylase forms malonyl-CoA in a regulated ATP-dependent committed step.
- NADPH
- NADPH supplies reducing power for biosynthetic reactions including fatty acid synthesis.
- Triacylglycerol
- Triacylglycerol is a hydrophobic storage lipid containing three fatty acyl groups on glycerol.
- Phospholipid
- A phospholipid is an amphipathic membrane lipid with hydrophobic tails and a polar head-group.
Fatty Acid and Complex Lipid Synthesis FAQ
Why is citrate exported?
Mitochondrial acetyl-CoA cannot freely cross the inner membrane. Citrate carries acetyl carbon outward, allowing cytosolic recovery for fatty acid synthesis while preserving compartment control under fed conditions.
What does malonyl-CoA do?
It provides activated carbon for chain elongation and helps restrain long-chain fatty acid entry into mitochondria. Those roles coordinate synthesis with suppression of opposing beta-oxidation.
Why does synthesis use NADPH?
Fatty acid synthesis is reductive and needs electron donation during elongation. NADPH serves that biosynthetic role, whereas NADH more commonly carries electrons from catabolism toward respiratory energy conversion.
How are storage and membrane lipids distinguished?
Triacylglycerols are neutral, hydrophobic energy stores with three acyl chains. Phospholipids are amphipathic molecules whose polar heads and hydrophobic tails support bilayer formation in cellular membranes.
Exam move
Draw synthesis and beta-oxidation in separate compartments, then compare carbon donor, carrier, energy direction and hormonal state. Put malonyl-CoA at the centre because it links committed synthesis to restraint of mitochondrial entry. Practise explaining why abundant acetyl-CoA is insufficient when ATP or NADPH is limited.
Follow newly made fatty acids into palmitate, elongation or desaturation, then choose triacylglycerol or phospholipid from the cell’s purpose. For data questions, state the denominator for lipid abundance and separate absolute synthesis from composition, remodelling and degradation. Use labelled incorporation when the claim concerns new synthesis rather than total lipid amount.
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